Low-frequency vibrations influence the surface quality of a machined workpiece considerably.
The numerical control of a machine tool controls the machining of a workpiece on the basis of a parts program in which a machining process is defined in many different machining cycles. A tool must follow a predefined path as precisely as possible, so that the form of the finished workpiece corresponds to the desired specifications. To that end, the various axes of the machine tool with their respective rotary or linear drives must be controlled accordingly.
In order to be able to adhere to a predefined machining path, closed-loop control structures are used which, in a position controller, calculate from the respective predefined setpoint position and the actual position of a machine axis, a setpoint speed (for linear drives) or setpoint rotational speed (for rotary drives), with which a position deviation is then intended to be corrected, if necessary. The difference between the setpoint speed and the actual speed is converted in a speed controller into a setpoint current which, multiplied by the motor constant of the drive, also corresponds to a setpoint torque of the drive. From this setpoint current—after comparison with the actual current—a setpoint voltage is computed in a current controller and is implemented in the drive amplifier and applied to the phases of the motor. Suitable measuring systems check the actual position of the respective drives, from which in each case the actual speed may be derived. Current sensors in the leads to the motor detect the actual current.
The connection between drive and tool is never completely rigid. On the contrary, it includes flexible, e.g., vibratory components. Therefore, mechanical resonances occur which, in the event of poor parameterization of the closed-loop control structure and/or low self-damping of the flexible components, may lead to unwanted vibrations. Due to the demand for increasingly higher bandwidth of the closed-loop control structures, achieved primarily by high amplification factors in the position control loop, such low-frequency resonant frequencies are also amplified and interfere with the tool path. Low-frequency vibrations in the range up to approximately 50 Hz are clearly visible as unwanted surface waviness in the machined workpiece.
Taking effect particularly negatively in the formation of such resonant vibrations is a negative phase rotation, as comes about especially due to the decelerations of the controlled system in interaction with the integral component of the speed controller. By reducing the corresponding amplification factor, the integral component may be reduced, and therefore the resonant vibration may be attenuated. At the same time, however, the rigidity of the machine tool and the quality of the disturbance correction decrease, as well.
European Patent No. 1 439 437 describes a closed-loop control structure for positioning a load with the aid of an electric motor, which has a device for the active damping of unwanted, low-frequency vibrations. The closed-loop control structure has a position controller, a speed controller and a current controller. In addition, damping signals which counteract unwanted, low-frequency vibrations are formed in the control loop. According to arrangement shown in FIG. 3 of European Patent No. 1 439 437, from a single sensor signal which includes the disturbing vibration, a first and a second damping signal of different phase angle are formed and are injected between the position controller and speed controller, to thus actively damp the interfering vibrations. Since according to this arrangement, the damping signals are obtained from the signals present in the control loop, external sensors which, for example, detect vibrations in the vicinity of the tool cannot be used.